The present invention relates to the art of magnetic resonance spectroscopy. It finds particular application in conjunction with in-vivo examinations and will be described with particular reference thereto. However, it is to be appreciated that the invention may find further application in conjunction with the spectroscopic examination of localized regions for imaging, chemical shift analysis, and the like.
When examining a complex structure, such as a region of a human patient, there are numerous different chemical compounds within the magnetic resonance examination region. To isolate a small volume of the region of interest, or voxel, two techniques are commonly employed--phase encoding techniques and voxel localization techniques. In phase encoding techniques, phase encoding gradients are applied across the sample such that phase can be used to obtain spatial encoding. In voxel localization, the signal is only recovered from a small volumetric element, or voxel.
More specifically, phase encoding techniques typically provide spatial definition over a region whose size is defined by the sensitive volume of the receiver coil. Because this sensitive volume is determined by the receiver coil geometry, it is substantially fixed from study to study. There is little latitude for defining or adjusting the region over which phase encoding occurs. This presents difficulties for spectroscopy because no allowance is made for avoiding regions of large magnetic field inhomogeneities, such as the boundaries between materials with different susceptibilities. These field inhomogeneities degrade water suppression and spatial resolution. Also, it is often desirable to avoid certain regions of the sample. For example, if fat layers are included in the sensitive volume, they can obscure the spectra due to "leakage" of the point-spread function between neighboring voxels. Fat signals are also much larger than the metabolites commonly of interest. Thus, the fat signals can degrade the dynamic range required to detect low concentrations of metabolites.
Voxel localization techniques, such as the technique described in U.S. Pat. No. 4,771,242, recover the signal only from a small voxel in the sample. To map or measure chemical concentrations over an extended region, a plurality of single voxel experiments are conducted. In each repetition, the voxel is defined at a different location within the sample. Although this technique is accurate, it tends to be time consuming.
In another voxel localization technique described in "In Vivo 1H NMR Spectroscopy of the Human Brain by Spatial Localization and Imaging Techniques", by P. Luyten, et al., SMRM Book of Abstracts, page 327 (1988), a volume is selected with refocused stimulated echoes. This volume selection technique utilizes a series of 90.degree. refocusing pulses. Phase encoding for one or two dimensional spectroscopic imaging is combined with the volume selection in order to suppress unwanted lipid signals from surrounding tissues. Although this technique enables subvoxels to be defined in the voxel, the Luyten technique has several drawbacks. First, the 90.degree. refocussing pulses only recover half the signal--the other half is lost. Further, spoiler gradient pulses for dephasing spurious echoes must be primarily the same polarity. Unlike opposite polarity spoiler pulses which provide for cancellation of gradient eddy currents, the unipolar spoiler pulses tend to promote eddy current degradation of the linewidths.
The present invention contemplates a new and improved spectroscopy technique which overcomes the above referenced problems and others.